1. Field of the Invention
The present invention relates to a thin film perpendicular magnetic recording head, their magnetic head fabrication process and magnetic disk drive for a highly reliable magnetic head with stable operation capable of generating a high magnetic recording field even on narrow tracks for high density magnetic recording.
2. Description of Related Art
In recent years, digitalization of diverse media has been making rapid progress along with advancements in information processing technology. Besides personal computers and servers, home appliance and audio devices must store huge amounts of digital information, creating an increasing demand greater than ever before for large capacity magnetic disk drives forming the core of non-volatile file systems. Large capacity disk drives in other words signifies recording on a medium with a higher bit density or in other words, a higher areal recording density.
A method called longitudinal magnetic recording is the generally used method for actual recording on magnetic disk drives. The longitudinal magnetic recording utilizes as a recording medium, a ferromagnetic layer possessing large magnetic coercivity in a direction parallel to the disk substrate surface, and records information by magnetizing the recording medium along the substrate area surface. In this case, an inverse magnetized section formed to face the longitudinal magnetization at a 180 degree angle is the bit 1.
In order to increase the longitudinal recording density, both the bit density towards the disk periphery (linear recording density) and the bit density radially along the disk (track density) must be simultaneously increased. Increases in the track density are limited by the pole width forming process for the read/write head and by the mechanism for positioning accuracy. However these factors are nothing more than technical issues. Increases in the linear recording density however are subject to basic restrictions due to the fact that the recording medium is an aggregate of ferromagnetic particles.
In the longitudinal magnetic recording method, magnetized sections that mutually oppose one another are mainly magnetic reversals. Near these magnetic reversals, large internal magnetic fields called demagnetization fields occur in a direction diminishing the magnetization. Transition areas or in other words, areas not having a high enough magnetic value are formed in a finite width in these magnetic reversals by the demagnetization fields.
Problems such as shifts in the actual position of the magnetic reversal occur when adjoining magnetic transition areas interfere with each other in locations when the bit length is short. These problems make it necessary to reduce the magnetic transition areas to at least a size smaller than the bit length. Increasing the linear recording density therefore requires a property on the medium where magnetization overcomes the demagnetization field. More specifically, along with improving the magnetic coercivity of the medium, the thickness of the magnetic recording layer must be reduced to suppress the demagnetization field.
The linear recording density is therefore greatly restricted by the magnetic properties and structure of the medium.
In the standard longitudinal recording, the ratio of linear recording density to track density is preferably about 5 to 10 times. To attain a recording density of 100 gigabits per square inch (1011 bits per square inch) based on this condition, the bit length towards should be made about 25 nanometers in the peripheral direction of the disk. However, estimating the properties required of a medium with a magnetic reversal width of 25 nanometers or less on a simple model, reveal that required conditions are a medium layer thickness of 15 nanometers or less and a magnetic coercivity of 5 kOe (oersted).
On the other hand, even under the precondition that the magnetic (recording) field generated by the write element in longitudinal recording has a saturated flux density (hereafter Bs) of 2.4 T (tesla) which is the maximum preferred level usable in a magnetic pole material, the upper figure will still be limited to 9 kOe. In this case, when the magnetic coercivity of the recording layer of the medium exceeds 5 kOe, obtaining a magnetic recording field strong enough to magnetize the medium is difficult. When the magnetic layer thickness of the cobalt alloy magnetic layer is below 15 nanometers, the actual volume of crystal grain becomes small so that the magnitude of the thermal energy (in other words, energy agitating the magnetization) can no longer be ignored compared to the anisotropic energy (in other words, energy for stabilizing the magnetization in a fixed direction) of the individual particles. The thermal fluctuation becomes drastic, causing the problem that thermal decay reduces the magnitude of the record magnetization as time passes. To suppress this thermal decay, the magnetic coercivity must be further increased or the volume of the crystal grains increased.
However as described above, there is an upper restriction on the allowable magnetic coercivity when the magnetic field of the head is limited. Furthermore, increasing the layer thickness in order to increase the volume of the crystal grains signifies an increase in the magnetic transition area due to an increase in the demagnetization field or in other words, means a drop in the allowable linear recording density.
However, attempting to attain a sufficient volume for the crystal grains longitudinally, increases the randomness of the magnetization distribution within the medium, leading to increased noise in the medium and preventing a sufficient S/N (signal-to-noise) ratio from being obtained. Therefore, achieving longitudinal recording in excess of an areal recording density of 100 gigabits per square inch while satisfying the conditions for thermal decay, low noise and sufficient recording is predicted to be basically difficult.
The perpendicular recording has been proposed to resolve these basic problems. The perpendicular (magnetic) recording is a method for magnetizing the thin-film layer in a direction perpendicular to the layer surface and its recording principle is basically different from the longitudinal recording of the related art. In the perpendicular (magnetic) recording, the particles are magnetized in a antiparallel configuration so adjacent magnetized particles are not made to face each, and therefore the perpendicular recording is not so affected by demagnetization fields. Perpendicular recording may therefore allow making the magnetic transition states extremely narrow and also make it easier to boost the linear recording density. Perpendicular recording can also be highly resistant to magnetic decay for the same reason, since the requirements for the medium thin-film as not as stringent as those for longitudinal recording.
As perpendicular magnetic recording is gathering attention as an ideal method for high density magnetic recording, mediums of various structures and materials combined with thin-film magnetic heads have been proposed. Perpendicular recording is composed of a method utilizing a single perpendicular magnetic layer; and comprised of a method forming adjacent flux keeper layers of low magnetic coercivity between the disk substrate and the perpendicular magnetic layer.
Perpendicular recording has the advantage that by utilizing a double layer perpendicular magnetic recording medium possessing a flux keeper layer and combining a single pole type write element (1): capable of reducing demagnetization field generated in a recording layer (2): a magnetic recording field can be generated having a steep distribution compared to the ring head utilized in longitudinal recording. This technology is for example disclosed in the non-patent document 1.
Mediums formed for example from a perpendicular magnetic layer of CoCR alloy formed on a flux keeper layer made from a soft magnetic layer such as permalloy or iron based amorphous alloy or fine crystallized alloy are under evaluation. In recent years, so-called granular mediums with fine particles of cobalt magnetic dispersed in SiO2 or superlattice layers such as Co/Pd or Co/Pt as the recording layer are also under evaluation. To stabilize magnetic domains of keeper layer, laminated layers combining with antiferromagnetic materials or magnetic multilayers which is composed of antiferromagnetically coupled ferromagnetic layers are for example being utilized.
The type of write element utilized in perpendicular recording with a perpendicular recording medium possessing a flux keeper layer is generally called a single-pole write element. This element does not use a structure of two poles facing each other via an extremely thin gap as does the so-called ring write element in longitudinal recording. Instead, the single magnetic pole (main pole) 13 as shown in FIG. 1 is characterized by a structure protruding towards the medium. To form a magnetic path however, a pole called an auxiliary pole 16 however is formed so as to put the coil 17 between them.
The auxiliary pole 16 forms a magnetic path in the path sequence of main pole 13, flux keeper layer 19, auxiliary pole 16, yokes 14, 15, and main pole 13 and is characterized in that recording can be performed with optimal efficiency. Since the magnetic flux flowing between the main pole 13 and the keep layer 19 cuts across the recording layer 18, the magnetic flux flow makes a magnetic recording field, and forms a record bit 20 in the recording layer 18.
The one serious problem unique to perpendicular recording utilizing the mutual effects of a single pole write element and magnetic flux keeper layer is the remanent magnetization of the main pole. This phenomenon is designated in non-patent document 2.
In this phenomenon called, “erase-after-write” (or erasing after write) disclosed in this document, the signal on the medium is erased by a direct current magnetic field due to remanent magnetization immediately after recording. The head in an actual magnetic disk drive is constantly moving above the disk. Therefore when this phenomenon occurs during operation, there is the possibility that data and servo information might be destroyed over an extremely wide range on the disk.
This phenomenon is a fatal defect in the reliability of the magnetic write-read system. One method to avoid this phenomenon described in patent document 1 is optimizing the shape of the yoke. This method could eliminate the problem of erasure occurring after writing in the yoke section due to remanent magnetization.
However, though there is a relatively high degree of freedom in designing the dimensions and shape of the yoke section, the pole tip which determines the width of the narrow recording track must be made small to meet the increased recording density. So it is necessary to employ a completely different means to suppress the remanent magnetization in the pole tip. One means is a method known in the related art utilizing a magnetic multilayer with a thin film (layer) of less than one micron in the main pole of the thin film magnetic head used for perpendicular recording.
A structure is disclosed in patent document 2 utilizing a magnetic multilayer in the main pole of the thin film magnetic head used for perpendicular recording. Methods are also disclosed in patent document 3, patent document 4, patent document 5 for utilizing optimal materials and layer structures to stabilize the magnetic domain in magnetic multilayers. However, these methods all have the objective of stabilizing the magnetic layer of a single magnetic domain and are inadequate or inapplicable as a means to prevent the erasure after write that is brought about by the single magnetic domain that results from making the magnetic pole smaller and narrower. The patent document 6 also discloses an example of a thin-film magnetic head utilizing a magnetic multilayer comprised of magnetic layers. However, this method can also be seen in the ring thin-film magnetic head utilized in longitudinal recording and was disclosed in technology to fix a magnetic domain for suppressing noise that accompanies changes in the structure of the magnetic domain during read operation. This structure is also different from the means for suppressing remanent magnetization in the pole tip after recording and is clearly not suitable.
The above disclosures assume as a precondition use of a material yielding comparatively satisfactory soft magnetic layer characteristics such as Ni—Fe, Fe—Ni alloy, and Fe. These disclosures are therefore unsuitable for high Bs material combinations exceeding 2.2 T such as Fe—Co alloy required for narrow tracks in the future.
[Patent document 1]
JP-A No. 291212/2001
[Patent document 2]
JP-A No. 324303/2002
[Patent document 3]
JP-A No. 54320/1993
[Patent document 4]
JP-A No. 195636/1994
[Patent document 5]
JP-A No. 135111/1995
[Patent document 6]
JP-A No. 49008/1991
[Non-patent document 1]
IEEE Transactions on Magnetics, Vol. MAG-20, No. 5, September 1984, pp.675–662, “Perpendicular Magnetic Recording-Evolution and Future”
[Non-patent document 2]
IEEE Transactions on Magnetics, Vol. MAG-32, No. 1, January 1996, pp. 97–102, “Challenges in the Practical Implementation of Perpendicular Magnetic Recording”
[Non-patent document 3]
The 198th Meeting of the Electrochemical Society, Meeting Abstracts, No. 582
In a perpendicular recording thin film magnetic head for high density recording in excess of 100 gigabits per square inch, a strong magnetic field in excess of 10 kOe must be generated from a narrow pole tip of 200 nanometers or less in width in order to write bits clearly on a magnetic recording medium with high magnetic coercivity of 5 kOe or more.
FIG. 2 is a graph showing the magnetic recording field distribution generated by a single pole type write element in the center of the recording track and computed by the 3-dimensional finite element method. The pole width was 150 nanometers to attain the required 140 gigabits per square inch. The four curves are respectively for a saturation flux density (Bs) of 2.4 T, 2.2 T, 2.0 T, and 1.6 T.
These results revealed that a ferromagnetic alloy mainly of Fe—CO with a high Bs of 2.2 T or more is required in the pole tip in order to generate a recording magnetic field in excess of 10 kOe at the write element for a narrow track having a high recording density in excess of 100 gigabits per square inch.
FIG. 3 is a graph showing results when many perpendicular recording thin film magnetic heads manufactured in different recording pole widths using high Bs materials of this type were subjected to 100 write-read repetitions and the degree of erase-after-write then calculated using the change in output as an indicator. The vertical axis is the change in output expressed in percent of average rated output over the 100 read-write cycles. The horizontal axis expresses the magnetic pole width of each head. The heads differ from one another only in the magnetic pole width and the other parameters are all fixed parameters.
As these results clearly show, virtually no erasure-after-write occurred in heads with a magnetic pole width of 200 nanometers or more, yet the extent of erasure-after-write suddenly increased on tracks narrower than 200 nanometers. The changes in output of below 10 percent observed on magnetic pole widths of 200 nanometers or more were confirmed as almost all being due to fluctuations in the sensitivity of the read element itself.
In the related art, erasure-after-write is thought to be caused by high recording efficiency from the combination of single pole type write element and keeper layer in the medium. In other words, remanent magnetization is not as likely to occur in independent write elements because demagnetization fields in the pole occur on the surface bearing to the medium. Therefore, the magnetic flux keeper layer in the medium here acts to reduce the demagnetization field in the magnetic pole having the effect that remanent magnetization is likely to occur.
The results in FIG. 2 however clearly show that this problem occurs more frequently on narrow tracks having a drop in recording efficiency. The erasure-after-write phenomenon here is therefore a mode different from the erasure-after-write disclosed in the reference documents. This is clearly due to a completely different physical phenomenon occurring within the write element.
Magnetization of ferromagnetic material can be considered the result of an aggregate of tiny magnetic momentum called spin. This spin has the constant effect of aligning the momentum of the vector in one direction by a mutual effect called exchange coupling. The ferromagnetic material however is processed in a limited size, so in order to prevent a vast increase in magnetostatic energy emitted to outer peripheral sections, the internal sections are separated into small areas known as magnetic domains.
These different magnetic domains need not always be made to face the same direction, and these domains are placed entirely in a magnetically closed structure. The boundaries of this magnetic domain are (magnetic domain) walls of a limited width. The size is determined by the magnetostatic energy versus the exchange coupling energy from adjoining non-aligned spins so that though differing by size and shape, ferromagnets widely known as comprised mainly by iron and cobalt have a size on the order of several hundred to some thousands of nanometers. Therefore, when the scale of the magnetic material is down to a few thousands of nanometers or less, a magnetic domain wall cannot be formed, and the magnet material tends to form into single domain states.
FIG. 4 shows results of the calculated remanent magnetization found by simulating the magnetic state of the magnetic pole tip. The vertical axis expresses stray magnetic fields due to remanent magnetization. The horizontal axis expresses the magnetic pole width. These results also reveal that the remanent magnetic field suddenly increases at pole widths of 200 nanometers and below.
FIGS. 5A and 5B are conceptual views of the magnetic state of the magnetic pole tip found by the above described simulation. The arrow 55 in the drawing indicates the direction of magnetization. When the magnetic pole width is as wide as 300 nanometers (FIG. 5A), the magnetic state is in a so-called closed domain structure. However when the magnetic pole width is a narrow 100 nanometers (FIG. 5B), one can see that the magnetic state is almost entirely a single domain and therefore a large remanent magnetic field is generated.
Examining the actual magnetic state of the write elements with 300 and 120 nanometer magnetic pole widths by spin-polarized scanning electron microscopy (SEM) shows as expected, that a with a wide pole width of 300 nanometers the magnetism is separated into numerous magnetic domains having various directions of magnetization. However, in the case of the narrow pole width of 120 nanometers, the magnetization is almost completely in a single domain state. These results allow concluding that the sudden increase in erasure-after-write observed in pole widths below 200 nanometers is due to the main pole tip preferring to be a single domain state.
In magnetic poles of this small size, remanent magnetization is easily prone to occur because of the tendency for uniform magnetization so that many cases of erasure-after-write have a high probability of occurring during device operation. In addition, in high Bs material such as Fe—Co alloy, the soft magnetic properties are generally inferior to those of typical soft magnetic materials such as Ni80Fe20. In other words, large hysteresis often appears in the magnetic curve due to the dispersion of crystalline magnetic anisotropy as well as materials having a large, positive magnetorestriction coefficient that are factors in inducing remanent magnetization and therefore erasure-after-write. This signifies that remanent magnetization is large when the hysteresis of the soft magnetic material is large and when there is no excitation. In thin film magnetic heads on the other hand, materials with a positive magnetorestriction coefficient are known to possess magnetic anisotropy induced in a direction perpendicular to the surface of the medium by an effect from anisotropic stress (generally called the inverse-magnetorestriction effect). (See the example in non-patent document 3.)
Magnetization is easily faced in a long-axis direction (shape magnetic anistrophy) due to the long, narrow shape of the four-cornered cylinder which is the original shape of the main pole tip. In addition, the crystalline magnetic anisotropy also becomes stronger in a direction perpendicular to the surface of the medium because of the inverse-magnetostriction. Therefore along with the single domain state that was previously described, the problem of erasure-after-write easily tends to occur due to remanent magnetization of the magnetic pole tip.
A structure is therefore required that essentially applies no stray magnetic field to the magnetic recording medium even remanent magnetization is generated perpendicular to the surface of the medium.
The inventors perceived that remanent magnetization can be suppressed in a thin film perpendicular magnetic recording head by employing a closed domain structure for the magnetic material even in magnetic poles of extremely small size by utilizing a magnetic multilayer structure and an optimal combination of materials capable of effectively eliminating the mutual exchange effect within the magnetic material causing a single domain state.